High power laser systems emitting short optical pulses are useful for many emerging applications in materials processing, biotechnology, instrumentation, metrology and medicine, representing one of the fastest growing laser markets nowadays. In particular, semiconductor laser based systems can benefit from the higher efficiency, compactness, wavelength tunability, lower cost, as well as the reduced complexity and footprint of laser diodes when compared to commercially available Ti:sapphire and fiber lasers.
The generation of optical pulses in semiconductor laser diodes is commonly achieved by means of gain switching, by driving the laser diode with an electrical injection of high peak current pulses having a duration of a few hundred picoseconds. The minimum pulse width that can be achieved with this technique is typically of a few tens of picoseconds which is in many cases longer than required by most applications. Besides, the average power is typically limited to a few mW due to the low duty cycle of the electrical pulse train used to forward bias the laser diode.
Passive mode locking is another well-established technique from which less than 10 ps optical pulses are commonly attained. It makes use of a two-section laser configuration used for separately pumping a gain and an absorber section. The technique has the advantage of only requiring DC sources without any fast electronics, therefore being a compact and cost-effective alternative.
However, in order to obtain short picosecond pulses, the two-section laser needs to be driven with high reverse absorber bias in the absorber section and low injection currents in the gain section, also resulting in very low average powers in the order of a few mW. Another factor severely limiting the maximum achievable peak power of the laser is catastrophic optical damage as a consequence of the increase in optical power density when short optical pulses propagate within the cavity.
Furthermore, as high peak power pulses are absorbed by the saturable absorber section, a high photocurrent is generated causing catastrophic damage of the absorber element. Additionally, fabrication of the laser diode requires extra processing steps as well as more challenging post-processing techniques, particularly if the laser is to be p-side down bonded for efficient heat dissipation. This arises from the small gap that is required to electrically isolate the gain from the absorber section within the laser contact layers.
In both techniques, the low average power level is not sufficient for the aforementioned applications, which are currently being addressed by costly Ti:sapphire or fiber lasers providing average powers in the Watt level or more. One way of increasing the output power of semiconductor pulsed laser diodes is through optical amplification, resulting in the so-called master-oscillator power amplifier configuration, typically consisting of several pre-amplification and amplification stages based on solid state or semiconductor active media. Besides being a complex solution which is less compact and cost effective, another disadvantage of this configuration arises when amplifying ultra-short optical pulses in the ps or fs range due to the very high peak power levels generated within the amplifier, leading to detrimental nonlinear pulse distortion and damage of the gain medium. This problem can be overcome through complex and sophisticated chirped pulse amplification techniques, in which the short optical pulses are first stretched before being launched into the amplifier and then recompressed at its output.